An imaging satellite is operated in order to investigate an atmospheric change, a weather forecast, a resource distribution, and the like, and the imaging satellite images a ground image at high resolution and transmits the imaged ground image to a base station while revolving on a low earth orbit. In this case, when an attitude of the satellite is not properly controlled, it is difficult to obtain an image of an intended point. Therefore, an attitude control system is mounted in the satellite to accurately perform an attitude control.
A performance index of the attitude control system of the satellite is indicated by directional awareness, directional precision, and the like. Examples of factors affecting the directional awareness, the directional precision, and the like include attitude control error, attitude determination error, orbit propagation error, alignment error of a payload such as an electro-optical camera attached to a body coordinate system, and the like, and therefore, improvement of the direction awareness and the directional precision is achieved by minimizing effects of the error factors.
The attitude determination error of the above-mentioned several error factors depends on characteristics, timing bias, mounting error, or the like of attitude control sensors such as a gyro sensor, a star tracking sensor, and the like, and in order to improve performance of the attitude control, an accurate calibration for the mounting error or characteristics of the above-mentioned sensors should be performed.
Examples of the attitude control sensor include a magnetic field sensor, a solar sensor, and the like, including the star tracking sensor and the gyro sensor, and the star tracking sensor and the gyro sensor among the above-mentioned sensors are sensors requiring high precision which are used for a precise attitude control, wherein the star tracking sensor performs a function of observing a position of a star with a plane image sensor to calculate an attitude of the sensor, and the gyro sensor performs a function of measuring angular velocity of a gyro, and the attitude and angular velocity measurement values calculated by the above-mentioned sensors are converted into attitude and angular velocity of the satellite by again considering a mounting attitude of each sensor for a satellite body to use for determining the attitude of the satellite.
Particularly, since directional information of image payload is important in a case of image satellite having high resolution, when the mounting attitude of the attitude control sensor is defined for a reference coordinate system of a satellite payload, not the body of the satellite, the attitude determination and the attitude control of the satellite are automatically performed for an attitude of the payload.
The most accurate measurement of the mounting attitude of the above-mentioned sensors is typically performed to be reflected to mounting software before launching the satellite, but error, that is, misalignment of the sensor from original mounting attitude information occurs due to impact during a process of launching the satellite, an environment change on the orbit, or the like as well as error upon the measurement. Therefore, in order to improve the directional awareness and precision of the satellite, the misalignment of the sensor should be necessarily again estimated and calibrated while a satellite is in orbit, after the launching of the satellite is completed.
The misalignment of the attitude control sensor is classified into relative misalignment and absolute misalignment, wherein the relative misalignment means relative misalignment between the sensors, and in order to match the attitude information measured by the respective sensors to each other, the attitude information should be calibrated, and the absolute misalignment means misalignment of the sensor for an image payload, and a calibration is required to accurately image a ground control point intended to be imaged or extract accurate position information using the imaged image.
A plurality of articles and studies for a technique for calibrating the relative misalignment and the absolute misalignment described above have been published, and the majority of studies for a method for calibrating the absolute misalignment among these mainly use a scheme in which the image payload is regarded as a kind of attitude control sensor such as the star tracking sensor to estimate an alignment attitude between the image payload and the attitude control sensor or a reference body.
However, as in a case of an imaging satellite having high resolution, in a case in which the image payload uses a scheme in which a linear array sensor is scanned and imaged, since an operation scheme is very different from the star tracking sensor using a two-dimensional plane sensor, it is difficult to directly apply the method for calculating the absolute misalignment as described above.